Metal oxide photocatalysts TiO.sub.2/MoO.sub.3 and TiO.sub.2/MoO.sub.3:V.sub.2O.sub.5 have been investigated by electron paramagnetic resonance (EPR) spectroscopy under in situ light excitation. Structural identification of all paramagnetic centers (PCs) recorded, such as surface and lattice Ti.sup.3+ ions as well as Mo.sup.5+ and V.sup.4+ ions and nitrogen .sup.14N atoms containing the unpaired electron ("N.sup.*-radicals"), have been performed. The temperature behavior of PCs in the range of 30-300 K and photoinduced changes of EPR spectra under light irradiation have been investigated. The validity of the Curie law was confirmed for the PCs of such composite photocatalysts. A new original method of detection of charge carrier separation and accumulation is suggested using EPR-technique. It is shown that TiO.sub.2/MoO.sub.3 and TiO.sub.2/MoO.sub.3:V.sub.2O.sub.5 photocatalysts are capable to accumulate photogenerated charge providing fresh possibilities for practical applications in photocatalysis because these oxide heterostructures retaining oxidation activity for a long time (more than 5 h) under the dark conditions after illumination. Graphical